Pinworm: A Common Human Parasite

US  Pharm. 2006;12:10-15.

Pharmacists who practiced in the 1970s may recall Povan, a red dye–derived medication once available for pinworm, and the seemingly endless number of prescriptions dispensed for it. This is in stark contrast to today's practice--prescriptions for anthelmintics to treat pinworm are rare. However, that does not necessarily mean that the prevalence of pinworm has decreased; since public awareness of this ancient parasite has dropped, it is less likely to be recognized when it is present.

Prevalence of Enterobiasis
Infestation with pinworm ( Enterobius vermicularis) is known as enterobiasis. This is the most common helminth infestation in the United States, with an estimated 42 million infested, translating to 14% of the population.^1,2 Investigators in other countries report much higher prevalences, such as 29% (Denmark), 39% (Thailand), 50% (England), and 61% (India).¹ 

Causative Agent
E. vermicularis is a white nematode (roundworm) with a length of 8 to 13 mm for the adult female (about the length of a standard staple) and 2 to 5 mm for the adult male.³ Its width is about 0.5 mm.¹ The ova are oval objects appearing microscopically to have three sides, approximately 55 x 25 mcm.¹

Life Cycle/Transmission
Humans are thought to be the sole hosts of E. vermicularis (see sidebar).^3,4 The common residence of the adult pinworm is the large intestine, where it attaches to the cecum and appendix.¹ Following mating, the gravid female must lay as many as 16,000 eggs.¹ They live for an average of eight to 13 weeks.^1,3 Although the reproductive habits of pinworm may be unpleasant to contemplate, pinworms are equipped with the requisite instincts to ensure survival of their species. The gravid mother will not lay eggs inside the host, since the ability to spread throughout the population would then be impaired. Rather, the female E. vermicularis has evolved the strategy of exiting the anal opening to deposit the eggs in the folds of perianal skin--a process known as ovipositing. Following ovipositing, females reenter the anal opening. Successful reentry is termed retroinfection.¹

Freshly laid Enterobius eggs are not immediately capable of infestation, as the larvae must develop first. The eggs embryonate (develop the ability to infest a patient) within four to six hours. ^1,3 If the environment is moist, they are potentially capable of infestation for 20 days. The newly laid eggs are surrounded by a sticky, albuminous material that allows them to adhere to the anal opening. To guarantee survival of the species, these eggs must reach another human's gastrointestinal tract, which can occur via several means.

An infested child might wear bed clothing that is sufficiently loose for the eggs to fall away as the host sleeps. If the eggs drop away at night, they could be found on sheets or blankets. The eggs are quite light and can be blown about by human activity, wind, or such activities as snapping egg-contaminated sheets while making the infested child's bed. Inhaled eggs may not enter the lungs, due to their weight and size, but may be deposited in the oropharynx, where they are easily swallowed.²

A second method of spread can occur during the day following ovipositing. The eggs can be easily removed from the anal skinfolds by scratching; the child then has viable eggs on his or her fingers or under the nails. As the child plays during the day, the eggs may be deposited on surfaces that other children's hands will have access to. Investigators visited homes and schools of infested children and identified pinworm eggs in 90% of samples taken from such diverse surfaces and locations as bars of soap, pet fur, swimming pools, walls of school dining rooms, and house dust samples. ⁵ If other children touching these surfaces subsequently place their fingers in their mouth to suck their thumb or bite their nails, the transmission loop is complete. Unfortunately, the child who originally had the infestation may also place his or her fingers inside the mouth, becoming reinfested and helping maintain a heavy parasite load--a process known as autoinfection.¹

Clothing worn the next day might also be loose enough to allow the eggs to fall away and out of the clothing as the child plays and carries out routine activities. The eggs could deposit on carpets, floors, or playground equipment, or in a sandbox. Again, placing fingers in the mouth allows transmission to occur.

After the patient is infested, pinworms must hatch and mature before ovipositing begins. The average time from ingestion to detection of eggs in the perianal areas is four to six weeks.¹

Epidemiology
Pinworm infestation is linked to age, being most common in children of school age, followed by preschool children.^6,7 Adults are the least common age-group to experience enterobiasis, with the exception of mothers whose children are infested.⁷

Children attending day care and other institutions are at greater risk for pinworm because of the high potential for transmission to those who are closely associated with the first case in the household. While some authors assert that pinworm crosses all social barriers, others state that poverty and poor personal hygiene increase the risk of infestation. ^1,8,9

Manifestations
Symptoms may take weeks or even months to become noticeable after the eggs are ingested.¹ While one third of those infested are asymptomatic, others begin to notice troubling vague and nonspecific symptoms.^1,10 The most common is anal, perianal, and/or perineal pruritus, stimulated by movement of the female, and albuminous substance that surrounds the eggs.^1,8,9,11 Itching worsens at night, when the female exits the anal opening.⁶ Other manifestations are anorexia, abdominal pain, irritability, disturbed sleep, insomnia, and restlessness.^1,7

Ectopic Enterobiasis
Retroinfection via anal reentrance does not occur via an organized search by the parasite; rather, the parasite crawls blindly until a body orifice is detected. The search is not always successful, and pinworms may enter other orifices, a process known as ectopic enterobiasis. Ectopic enterobiasis is less common in males, as the urinary opening is in a more distant location from the anal opening. However, ectopic male infestation may cause urethritis or epididymitis. Female physiology favors ectopic enterobiasis, as the vaginal and urethral openings are in closer proximity to the anal opening. After pinworms have gained entrance to the female genitourinary tract, they may cause vaginitis, vulvar irritation, endometriosis, salpingitis, urethritis, pelvic inflammatory disease, or pyelitis.^1,12 The patient may notice such symptoms as dysuria, enuresis, vaginal discharge, vulvovaginitis, or postmenopausal bleeding.¹

Prevention of Enterobiasis
Parents of patients infested with pinworm may ask the pharmacist how to prevent a future problem and how to maintain cleanliness during an ongoing episode. Several commonsense steps can be followed to decrease the risk of reinfection. Most require added attention to hygiene.^6,7 For instance, the patient should shower every morning to decrease the chance of spreading the parasite to others and of retroinfection. Showering is preferable to bathing, since soaking in the bath with eggs floating in the water could conceivably allow them to enter the genitourinary tract. Patients should be urged to change underwear and sleepwear each day and night. Each period of wear should be followed by thorough washing. While the CDC advises against the practice, sheets could be washed daily to remove eggs that fell onto them at night. If sheets are not washed every day, they should be pulled back, and all blinds and curtains should be opened during the day, as pinworm eggs are sensitive to sunlight. Nails should be trimmed short. Children and adults must strive to keep their fingers out of their mouths at all times. Hands must always be washed thoroughly after bowel movements, especially during a bout of enterobiasis. Hands should be washed before eating and after changing diapers. Children must be urged to stop scratching their anal area, to prevent contaminating their hands and to reduce their risk of excoriating the skin. Dressing them in close-fitting underwear and cotton gloves can reduce or stop nocturnal scratching. The floors should be vacuumed every other day, since eggs can survive for two weeks on the carpet. Children may be allowed to return to day care after the first treatment with pyrantel pamoate, if they have bathed and nails are trimmed and cleansed.

Recognition/Treatment
The patient leaflet describes steps to diagnose and treat enterobiasis. Patients may wish to provide a stool sample; however, pinworms are seldom found in stool specimens, as they are not usually released into the fecal stream.

The leaflet describes use of the only FDA-approved anthelmintic: pyrantel pamoate. The medication paralyzes the worm, causing it to lose its grip on the bowel lining and to be flushed from the body. Controversy surrounds the issue of FDA labeling versus recommendations from the CDC. The discrepancy stems from the fact that pyrantel pamoate is effective only for live worms and does not affect eggs or larvae. Thus, immature larvae and eggs that hatch after the first treatment remain fully viable and capable of beginning the infestation anew. The FDA-approved labeling states that the patient should take only one dose and take other doses only under the advice of a physician. However, the CDC states on a Web page devoted to patients: "With either prescription or over-the counter drugs.[sic] You should consult your health care provider before treating a suspected case of pinworm. Treatment involves a two-dose course. The second dose should be given 2 weeks after the first."⁷ This statement to patients effectively countermands and invalidates the FDA-approved labeling and gives tacit approval to the pharmacist to recommend a second dose.

Conclusion
Pinworm is a common but seldom-recognized condition. An informed pharmacist can help infested patients obtain appropriate treatment and prevent spread to other family members and playmates.

References
1. Burkhart CN, Burkhart CG. Assessment of frequency, transmission, and genitourinary complications of enterobiasis (pinworms). Int J Dermatol. 2005;44:837-840.
2. Tsibouris P, Galeas T, Moussia M, et al. Two cases of eosinophilic gastroenteritis and malabsorption due to Enterobius vermicularis. Dig Dis Sci. 2005;50:2389-2392.
3. Enterobiasis. Centers for Disease Control and Prevention. Available at: www.dpd.cdc.gov/dpdx/HTML/Enterobiasis.htm. Accessed October 24, 2006.
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5. Gale EA. A missing link in the hygiene hypothesis? Diabetologia. 2002;45:588-594.
6. What you need to know about threadworm. NursTimes. 2004;100:30.
7. Pinworm Infection. Centers for Disease Control. Available at: www.cdc.gov/NCIDOD/dpd/parasites/pinworm/factsht_pinworm.htm. Accessed October 24, 2006.
8. Otu-Bassey IB, Ejezie GC, Epoke J, et al. Enterobiasis and its relationship with anal itching and enuresis among school-age children in Calabar, Nigeria. Ann Trop Med Parasitol. 2005;99:611-616.
9. Singh SP, Panda C, Rout N, et al. Anal albendazole application for pruritus ani in threadworm infestation. J Trop Pediatr. 2005;51:386.
10. Petro M, Iavu K, Minocha A. Unusual endoscopic and microscopic view of Enterobius vermicularis: a case report with a review of the literature. South Med J. 2005;98:927-929.
11. Brown MD. Images in clinical medicine: Enterobius vermicularis. N Engl J Med. 2006;354:e12.
12. Jain S, Sharma P, Gupta R. Dual cervical parasitosis in a psychiatric patient. Cytopathology. 2005;16:53-54.
13. Chan OT, Lee EK, Hardman JM, et al. The cockroach as a host for Trichinella and Enterobius vermicularis: Implications for public health. Hawaii Med J. 2004;63:74-77.

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